General wearable devices may provide call services, for example, through specific wired or wireless headsets which are preliminarily prepared. The usability of wearable devices thus allows users to have hands free. However, a wearable device such as wired/wireless headset needs to be worn on a user for call service. Especially, in the case that a user is violently exercising, a wired/wireless headset could be burdensome to the user and separable from the user. Therefore, instead of an additional wired/wireless headset, it may be preferred to use a bone conduction speaker for a wearable device, which may highly facilitate a user.